1. Field of the Invention
The present invention relates generally to soft handover in an asynchronous code division multiple access (CDMA) mobile communication system, and in particular, to an apparatus and method for determining soft handover of a user equipment (UE) supporting a multimedia broadcast/multicast service (MBMS).
2. Description of the Related Art
Recently, due to developments in the communication industry, a service for multicasting multimedia communication for transmitting not only voice service data but also high-capacity data such as packet data and circuit data is being developed in code division multiple access (“CDMA”) mobile communication systems. In order to support the multicasting multimedia communication, a broadcast/multicast service is required in which one data source provides a service to a plurality of user equipments (hereinafter referred to as “UEs”). The broadcast/multicast service can be divided into a cell broadcast service (hereinafter referred to as “CBS”), which is a message-based service, and a multimedia broadcast/multicast service (hereinafter referred to as “MBMS”) supporting multimedia data, such as real-time image and voice, still image, and text.
CBS refers to a service for broadcasting a plurality of messages to all UEs located in a particular service area. The particular service area where the CBS is provided can be the entire area where the CBS is provided in one cell. MBMS, supporting both voice data and image data, requires a great number of transmission resources. Therefore, the MBMS is provided over a broadcasting channel, because of the possibility that a plurality of services would be simultaneously provided in one cell.
Generally, a UE does not stay within one cell area, but travels from one cell area to another cell area. In this case, an existing voice service or data service can be continued through soft handover. Handover during data transmission/reception can be roughly classified into handover in a CELL_DCH state and handover in a CELL_FACH state.
Handover in the CELL_DCH state occurs when a dedicated channel (“DCH”) is set up. Describing a handover procedure in the CELL_DCH state, a UE measures a common pilot channel (hereinafter referred to as “CPICH”) and sends a measurement report to a servicing radio network controller (hereinafter referred to as “SRNC”). The SRNC determines from the measurement report from the UE whether to perform handover, and sends a handover command to the UE. Therefore, soft combining can be performed by combining the measurement report from the UE and the handover command from the SRNC.
Handover in the CELL_FACH (“Forward Access Channel”) state happens when a common channel is set up. Describing the handover procedure in the CELL-FACH state, a UE determines a best cell by itself by measuring CPICH and performs cell update. In this case, there is no necessity to send a measurement report to an SRNC, but it is not possible to perform soft combining. For these reasons, the MBMS service, for which handover in the CELL_FACH state must be considered since it is serviced over a broadcasting channel, faces a soft combining problem.